FIG. 1 represents an exemplary conventional amplifier AMPo of a radio frequency reception chain.
An input terminal 122 receives a voltage radio frequency signal, amplified by a transconductance amplification stage TRSC comprising two cascoded amplification arrangements 101, 111 and whose current output 120 exhibits a controlled common mode, so as, for example, to facilitate its interfacing with other blocks.
The first cascoded amplification arrangement 101 comprises a transconductance amplification transistor 103 of P-MOS type whose gate is coupled to the input terminal 122, in series with a cascode transistor 105 of P-MOS type whose drain is connected to the output 120.
The second cascoded amplification arrangement 111 comprises a transconductance amplification transistor 113 of N-MOS type whose gate is coupled to the input terminal 122, in series with a cascode transistor 115 of N-MOS type whose drain is connected to the output 120.
A resistor 109 (of value R2) connected between a supply voltage terminal VDD and the amplification transistor 103 makes it possible to perform the measurement of the static current which passes through the cascoded amplification arrangements 101, 111 of the transconductance amplification stage TRSC, and flows towards a terminal intended to be connected to a reference voltage GND, for example, the ground.
Such controlled common-mode configurations are employed notably within the framework of the reception of radio frequency signals, in particular with low energy consumption, because of constraints in respect of low noise and of high operating frequencies.
However, the biasing of the common output terminal 120 is not controllable in such a cascoded amplification stage TRSC alone.
Closed-loop feedback control systems (BCLI, BCLMC) have been produced, the first controlling the flow of the static current in both of the cascoded amplification arrangements 101, 111, the second so as to control the common-mode bias potential of the output terminal 120.
A closed-loop current feedback control BCLI comprises in this representation a reference current Iref generator 131, a first current mirror arrangement 133 and a second current mirror arrangement 135.
The current-feedback-control loop BCLI makes it possible to compare the voltage across the terminals of the resistor 109, representative of the static current flowing in the cascoded amplification stage TRSC, with a reference voltage generated by the flow of the reference current I in a resistor 137.
A lack or an excess of static current will be compensated by respectively an increase or a decrease of the voltage controlling the amplification transistor 113 by way of a resistor 117.
Moreover, closed-loop feedback control of the biasing of the common mode BCLMC, makes it possible to compensate a drop or a rise of the bias voltage of the output 120.
The common-mode feedback control loop BCLMC comprises a probing transistor 141 controlled by the potential present on the common output 120, a transistor 143 controlled by a fixed reference voltage present on a reference voltage terminal 145, each being connected on the one hand to the supply voltage terminal VDD and on the other hand to the reference voltage terminal GND by way of a current generator 149 drawing a current Iref in the sources of the transistors 141 and 143.
A current mirror arrangement 147 makes it possible to produce the difference of the currents passing through the transistors 141 and 143 and to compensate a drop or a rise in the bias of the output 120 by lowering or by increasing the voltage controlling the P-MOS transistor 103, by way of a resistor 107.
Consequently, the feedback control of one loop (BCLMC or BCLI) acts strongly on the reaction of the other loop (BCLI or BCLMC). This interaction of one loop on the other can be controlled in terms of stability of a feedback-controlled system only if one of the 2 loops exhibits a very low cutoff frequency with respect to the other.
The necessity for one of the 2 loops to be very slow introduces a long and very undesirable overall response time of the amplifier AMPo.
FIG. 2 represents a prior art solution in which the closed-loop feedback control of the biasing of the common mode BCLMC is replaced with a self-biasing circuit MCo for the common mode.
The self-biasing circuit MCo comprises a resistor 142 connected between the gate of the amplification transistor 103 and the output terminal 120.
The resistor 142 has a high value so as to minimize the losses in the output current and the losses in terms of gain of the amplification stage TRSC.
The bias of the output terminal is applied notably on the basis of the voltage present between the source and the gate of the amplification transistor 103.
This solution exhibits the drawback of being very unstable in relation to temperature variations (the gate-source voltage of the amplification transistor possibly varying by 2 mV/° C.) and of being unstable because of the variations of the characteristics of the amplification transistor 103 from one fabrication batch to another.
The bias voltage thus undergoes detrimental variations due to parasitic and uncontrollable variations of the voltage present between the gate and the source of the amplification transistor.
This is why according to embodiments an amplification device comprises a cascoded amplification stage, a current feedback control closed loop and a common-mode biasing open loop, the common mode being self-biased and furthermore self-regulated in relation to parasitic variations, by means of the open loop.
By cascoded amplification arrangement is meant an arrangement comprising a transconductance amplification transistor and a so-called cascode transistor in series. The role of the cascode transistor is, for example, to afford good input-output isolation, a high output impedance, a gain or a greater bandwidth.